1. Field of the Invention
The present invention relates to an inkjet printing apparatus, an image processing method, and an image processing apparatus. More specifically, the present invention relates to a technique to reduce an adverse effect to an image caused in overlapping (joint printing) boundary portions of mutually adjacent printing regions.
2. Description of the Related Art
The joint printing in inkjet printing is performed for the following purpose.
For example, a line printer employing a line-type inkjet head in which arrayed are numerous ink ejection openings in an orthogonal direction to a direction of printing medium conveyance can speed up the image formation. However, it is difficult to manufacture such a line-type inkjet head as provided with the ink ejection openings, fluid channels communicating therewith, and elements for generating energy to be used for ejection (such constituents will be hereinafter collectively referred to as nozzles when appropriate) over a broad range such as an entire width of a printing medium, with no defects. Accordingly, a head formed by arraying multiple short-sized head chips, which are relatively easy to manufacture and inexpensive, so as to extend the total length to satisfy a desired dimension is used for the line-shaped inkjet head.
However, in this configuration, an image defect called a “white stripe” is apt to occur along a sub-scanning direction in a joint section between the head chips which are used for printing mutually adjacent printing regions. This aspect occurs due to a so-called “end-deviation”. The end-deviation is a phenomenon that a direction of ink ejection from ejection openings located on an end of the head chip is deflected inside an array of ejection openings in the head chip by an influence of airflow generated between the head chip and a printing medium in the case of printing an image requiring a heavy printing duty at a high speed. For this reason, no matter how accurately the head chips are arranged or even when end portions of the head chips overlap one another and the facing edges of the used areas of the ink ejection openings of these two head chips are aligned in the overlapping sections, white stripes may still occur at boundary sections between adjacent printing regions corresponding to the overlapping sections of the head chips. In addition, a stripe-shaped image defect may also occur at the boundary sections between adjacent printing regions in the case where the head chips have different amounts of ejection or where the head chips have different ink ejection speeds which causes a phenomenon that the ejected ink arrives at a printing medium with different timing, both of which are attributable to piece-to-piece manufacturing variations of the head chips.
Accordingly, as disclosed in Japanese Patent Laid-Open No. 5-57965 (1993), for example, it is possible to reduce stripe-shaped image defects by disposing head chips so as to allow end portions overlap one another in the sub-scanning direction and appropriately determining printing densities of the head chips corresponding to joint sections.
Moreover, the above-mentioned stripe-shaped defects also occur in so-called serial printers configured to perform printing by alternately repeating main scanning in a direction different from a direction of an array of ink ejection openings in a printing head formed by arraying the ink ejection openings and conveyance (sub-scanning) of a printing medium. Specifically, when a width of a printing region (a band) in one session of main scanning is narrowed by the “end-deviation” or when the sub-scanning amount is excessive, a section where printing is not executed is left at a band boundary section along the main scanning direction. Therefore, it is effective to overlap band edges each other by setting sub-scanning amount at shorter than a length of an array of the ink ejection openings.
Development of a large white stripe is reduced by applying the technique disclosed in Japanese Patent Laid-Open No. 5-57965 (1993), for example. However, the inventors of the present invention have found out that a joint section has a lower gloss level than a non-joint section and an adverse effect to an image is caused as a consequence. Specifically, a section having a lower gloss level emerges as a stripe extending in the sub-scanning direction in the case of a line printer or as a stripe extending in the main scanning direction in the case of a serial printer, which is observed as uneven gloss. Moreover, the inventors have found out that the uneven gloss is observed more remarkably in the case of printing on a printing medium such as gloss paper by use of pigment ink. Normally, gloss paper is selected when a user desires high-quality printing. Thus the fact that such an adverse effect to an image is caused on gloss paper does not appropriate to the needs of the user and therefore make it meaningless to use quality paper. Accordingly, this is a serious problem.